The present invention pertains to a method and apparatus for deprioritizing a high priority client. More particularly, the present invention pertains to a method of improving the efficiency in handling isochronous data traffic through the implementation of a deprioritizing device.
As is known in the art, isochronous data streams are time-dependent. It refers to processes where data must be delivered within certain time constraints. For example, multimedia streams require an isochronous transport mechanism to ensure that the data is delivered as fast as it is displayed and to ensure that the video is synchronized with the display timing. An isochronous data stream request is generally referred to as a “high priority” client. These high priority requests are sensitive to time, such that a certain amount of data must be retrieved within a certain amount of time.
Within an integrated chipset graphics system, large amounts of high priority data are constantly retrieved for display on a computer monitor (e.g. an overlay streamer requesting isochronous data). The lower priority client may, for example, be the central processing unit (CPU). This high priority client has certain known characteristics. The client fetches certain types of pixel data, which will eventually be displayed on the computer monitor. A large grouping of scanlines creates a 2-dimensional image that results in a viewable picture on a computer monitor. The behavior of the monitor is such, that one horizontal scanline is completely displayed before the monitor starts to display the next scanline. In addition, there exist screen timings that determine how long it takes to display the given scanline. The scanline itself also contains a fixed amount of data. Therefore, in order that there not be any corruption on the screen (i.e. the computer monitor displays garbage data), the pixels of the scanline must be fetched and be available to be displayed before the time that the screen is ready to draw the pixels. If a pixel is not yet ready, because the screen timings are fixed, the monitor will display something other than the expected pixel and move on with drawing the rest of the scanline incorrectly.
For this reason, all of the data for the current scanline is already available, fetched prior to being displayed, so that there will be no screen corruption. Typically, a First-In First-Out (FIFO) device is implemented to load the data of the request from memory (either from the cache, main or other memory). The data is then removed from the FIFO as needed by the requesting client. When the amount of data within the FIFO goes below a certain designated watermark, a high priority request is sent out to fill the FIFO again. However, there are instances when an isochronous streamer is fetching data that will not be needed for a considerable amount of time. The fetching of this data will cause increased latencies on lower priority clients making requests for data. For example, the higher priority of the isochronous streamer request will likely obstruct the lower priority requests of, for example, the CPU. All overlay requests are high priority, and as such, use up all available memory bandwidth. The CPU must then wait for the streamer's isochronous request to be fulfilled before it is serviced, although the data is not immediately needed for display. This aggressive fetching induces long latencies on the CPU, thereby decreasing overall system performance.
In view of the above, there is a need for a method and apparatus for deprioritizing a high priority client to improve the efficiency in handling data traffic requests from both high priority and lower priority clients.